linux-kernel - Re: [PATCH v4 11/12] x86/mm: enable AMD translation cache extensions

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Message-ID: <09807556-46fe-46ae-ae3f-7083a1b12253@citrix.com>
Date: Mon, 13 Jan 2025 11:32:00 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: riel@...riel.com
Cc: akpm@...ux-foundation.org, bp@...en8.de, dave.hansen@...ux.intel.com,
 jannh@...gle.com, kernel-team@...a.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, nadav.amit@...il.com, peterz@...radead.org,
 thomas.lendacky@....com, x86@...nel.org, zhengqi.arch@...edance.com
Subject: Re: [PATCH v4 11/12] x86/mm: enable AMD translation cache extensions

> diff
> <https://lore.kernel.org/lkml/20250112155453.1104139-1-riel@surriel.com/T/#iZ2e.:..:20250112155453.1104139-12-riel::40surriel.com:1arch:x86:kernel:cpu:amd.c>
> --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c index
> bcf73775b4f8..b7e84d43a22d 100644 --- a/arch/x86/kernel/cpu/amd.c +++
> b/arch/x86/kernel/cpu/amd.c @@ -1071,6 +1071,9 @@ static void
> init_amd(struct cpuinfo_x86 *c)  
>  	/* AMD CPUs don't need fencing after x2APIC/TSC_DEADLINE MSR writes. */
>  	clear_cpu_cap(c, X86_FEATURE_APIC_MSRS_FENCE);
> + + if (cpu_feature_enabled(X86_FEATURE_INVLPGB)) +
> msr_set_bit(MSR_EFER, _EFER_TCE);  }
>  
>  #ifdef CONFIG_X86_32

I don't think this is wise.  TCE is orthogonal to INVLPGB.

Either Linux is safe with TCE turned on, and it should be turned on
everywhere (it goes back to Fam10h CPUs IIRC), or Linux isn't safe with
TCE turned on, and this needs to depend on some other condition.

Or, is this a typo and did you mean to check the TCE CPUID bit, rather
than the INVLPGB CPUID bit?

~Andrew